Wind power plant frequency control

ABSTRACT

A method of operating a wind power plant, with at least one wind turbine generator connected to an electrical grid is provided. The method includes monitoring a frequency parameter and comparing it with a frequency set point, as to select a low or high frequency event and determining a power reference to the a least one wind turbine generator, based on the frequency parameter. In case a high frequency event is detected, the power reference is determined as a minimum of a selection of at least a first and a second minimum power reference. And in case a low frequency event is detected, the power reference is determined as a maximum of a selection of at least a first and a second maximum power reference, dispatching the power reference to the at least one wind turbine generator.

FIELD OF THE INVENTION

The present invention relates to a method of operating a wind powerplant, with at least one wind turbine generator connected to anelectrical grid, and controlling the electrical frequency of theelectrical grid. The invention also relates to a wind power plantoperating according to the method.

BACKGROUND OF THE INVENTION

In an electrical utility grid consumers can usually consume electricpower in an uncontrolled manner. Since hardly any energy is stored inthe grid, there can be no imbalance between the power produced and thepower consumed. Therefore, the momentary production of power shall matchthe momentary power consumption. Overproduction leads to an increase ofthe grid frequency beyond the nominal value (e.g. 50 or 60 Hz), sincethe conventional synchronous generators accelerate, while overconsumption will lead to a decrease of the grid frequency beyond thenominal value (e.g. 50 or 60 Hz), since the conventional synchronousgenerators will then decelerate.

In order to stabilize the frequency of the electrical grid,conventionally about 10% of the producers contribute to what is called“primary power control”. These producers, also referred to as “primarycontrollers”, increase power output when the frequency falls below thenominal value and decrease power output when it rises above the nominalvalue.

Conventionally, wind turbine generators do not contribute to primarycontrol, firstly because wind turbine generators can not normallyincrease their output power by command (as they normally operate atnominal load or, when operating at partial load, at an optimal workingpoint), and secondly because the available wind power shall normally beentirely exploited.

Generally, wind power adds an additional moment of grid instabilitybecause, with a significant fraction of wind power in a grid, not onlythe consumption is uncontrolled, but also the production by wind turbinegenerators. Even though wind forecasts enable the wind power productionto be predicted with considerable accuracy on a long-term basis (at thelevel of hours), the wind speed normally fluctuates in an unpredictablemanner on a short-term basis (at the level of minutes down to a fewseconds). A wind turbine generator operating at partial load (i.e. whenthe wind speed is below the nominal wind speed of the wind turbinegenerator considered) will normally transform these wind-speedfluctuations into corresponding fluctuations of the amount of real powerproduced and supplied to the electrical grid. Only at wind speeds abovenominal, when a wind turbine generator operates at nominal load, windturbine generators normally control their output power to be constant atthe nominal output power.

The consequence of fluctuating-power production by wind turbinegenerators on the grid stability depends on characteristics of the grid.In a large, stable grid a power fluctuation by a wind turbine generatoror wind power plant will not produce any significant response in theform of a frequency fluctuation. Thus, such grids can cope with higherpower variations.

In order to ensure a stable electrical grid, various requirements havebeen set for grid connection of wind turbine generators. Topics of thegrid codes may vary, but often there are requirement for Voltage controland Frequency control. Different grid codes may require differentcharacteristics for frequency response from wind power. Each grid codemay have pros and cons, depending on the characteristics and needs ofthe particular power system.

SUMMARY OF THE INVENTION

This Summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used as an aid in determining the scope of the claimed subjectmatter.

In an aspect, the present invention relates to a method of operating awind power plant, with at least one wind turbine generator connected toan electrical grid, wherein the method comprises the steps of:

-   -   monitoring a frequency parameter and comparing it with a        frequency set point, as to select a low or high frequency event;    -   determining a power reference to the a least one wind turbine        generator, based on the frequency parameter, wherein:        -   In case a high frequency event is detected, the power            reference is            -   determined as a minimum of a selection of at least a                first and a second minimum power reference,        -   In case a low frequency event is detected, the power            reference is            -   determined as a maximum of a selection of at least a                first and a second maximum power reference,    -   dispatching the power reference to the at least one wind turbine        generator.

An advantage of first aspect is mainly that an advantage of the presentinvention is that there is a limit to the response and also that anincrease or decrease due to variation in the wind will not act againstthe frequency control.

According to one embodiment of the invention the first minimum powerreference is a pre-event electrical power output subtracted a poweramount proportionally to the grid frequency deviation ΔP_(FC), andwherein the second minimum power reference is a pre-event curtailedelectrical power reference subtracted a power amount proportionally tothe grid frequency deviation ΔP_(FC).

An advantage of this embodiment is that the limit to the response isproportional to the grid frequency deviation.

According to one embodiment of the invention the first maximum powerreference is a summation of a pre-event electrical power output and apower amount proportionally to the grid frequency deviation ΔP_(FC), andwherein the second maximum power reference is a summation of a curtailedelectrical power output and a power amount proportionally to the gridfrequency deviation ΔP_(FC).

An advantage of this embodiment is that the limit to the response isproportional to the grid frequency deviation.

According to one embodiment of the invention the curtailed electricalpower reference is a power reserve.

An advantage of this embodiment is that the curtailed electrical canhelp support the grid.

According to one embodiment of the invention the power reserve iscombined with a limitation function, as to reduce power fluctuationsfrom the wind power plant.

An advantage of this embodiment is that the limitation function preventspower fluctuations.

According to one embodiment of the invention the power amountproportionally to the grid frequency deviation, ΔP_(FC) furthercomprises a threshold component.

An advantage of this embodiment is that the threshold work as adeadband.

According to one embodiment of the threshold component consists of a lowfrequency threshold value and a high frequency threshold value.

An advantage of this embodiment is that the threshold work as a deadbandwith an upper and lower limit.

In a second aspect, the present invention relates to a wind power plant,with a least one wind turbine generator connected to an electrical grid,and a power plant controller, wherein the power plant controller isarranged to monitor a frequency parameter and compare it with afrequency set point, as to select a low or high frequency event, andsaid power plant controller is arranged to determine a power referenceto the at least one wind turbine generator, based on the frequencyparameter, wherein:

-   -   In case a high frequency event is detected, the power reference        is        -   determined as a minimum of a selection of at least a first            and a second minimum power reference,    -   In case a low frequency event is detected, the power reference        is        -   determined as a maximum of a selection of at least a first            and a second maximum power reference,    -   and said power plant controller is arranged to dispatch the        power reference to the at least one wind turbine generator.

The advantages of the second aspect and its embodiments are equivalentto the advantages for the first aspect of the present invention.

The individual aspects of the present invention may each be combinedwith any of the other aspects. These and other aspects of the inventionwill be apparent from the following description with reference to thedescribed embodiments.

Any of the attendant features will be more readily appreciated as thesame become better understood by reference to the following detaileddescription considered in connection with the accompanying drawings. Thepreferred features may be combined as appropriate, as would be apparentto a skilled person, and may be combined with any of the aspects of theinvention.

BRIEF DESCRIPTION OF THE FIGURES

The power system and its method according to the invention will now bedescribed in more detail with regard to the accompanying figures. Thefigures show one way of implementing the present invention and is not tobe construed as being limiting to other possible embodiments fallingwithin the scope of the attached claim set.

FIG. 1 shows a wind turbine generator according to the present invention

FIG. 2 shows introduction of power reserve.

FIG. 3 shows example of coexistence of power reserve and fluctuationlimitation.

FIG. 4 shows frequency response characteristic.

FIG. 5 shows calculation of the frequency response characteristic.

FIG. 6 shows Pre-event values for frequency response.

FIG. 7 shows calculation of wind power demand for frequency response.

FIG. 8 shows a frequency response characteristic for a high-frequencyevent.

FIG. 9 shows a frequency response characteristic for a low-frequencyevent.

FIG. 10 shows examples of the numerical values of the electricalfrequency requirements.

FIG. 11 is a schematic flow chart of an embodiment of the method.

DETAILED DESCRIPTION OF AN EMBODIMENT

The present invention will now be explained in further details. Whilethe invention is susceptible to various modifications and alternativeforms, specific embodiments have been disclosed by way of examples. Itshould be understood, however, that the invention is not intended to belimited to the particular forms disclosed. Rather, the invention is tocover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the appended claims.

The individual elements of an embodiment of the invention may bephysically, functionally and logically implemented in any suitable waysuch as in a single unit, in a plurality of units or as part of separatefunctional units. The invention may be implemented in a single unit, orbe both physically and functionally distributed between different unitsand processors.

The embodiments of the present invention pertain to a power system witha plurality of wind turbine generators (e.g. a variable-speed windturbine generator). The power system seeks to provide frequency supportto the electrical grid to which it is connected.

The wind turbine generator (e.g. a variable-speed wind turbinegenerator) which supplies power to an electric grid may be equipped withother regulation capacity against grid-frequency and active powerfluctuations. “Electric grid” or “grid” is a utility grid outside theboundary and point of common coupling of a wind power plant; whenreference is made to the grid within a wind power plant an expressionwith explicit indication to the wind power plant is made, e.g.,“wind-park grid”. Regulation capacity against grid-frequencyfluctuations is, e.g., provided by a certain fraction (typically about10%) of primary controllers, which are typically conventional producers,which may use steam- or gas-driven turbines and fossil energy sources,or hydropower). The primary controllers increase power output when thefrequency falls below the nominal value (e.g. 50 or 60 Hz) and decreasepower output when it rises above the nominal value.

As the present text deals with active power rather than reactive power,active power is briefly referred to as “power”, or “output power”. Wherereactive power is addressed, it is explicitly referred to as “reactivepower” There is an upper limit to the output power which can be producedby the wind turbine generator according to the embodiments, e.g. due tostructural limits and a current limit in the wind turbine generator'selectric converter. This amount of power is referred to as “nominalpower”. The wind speed sufficient for the wind turbine generator toproduce the nominal power is referred to as “nominal wind speed”. Whenthe wind turbine generator according to the embodiments operates at windspeeds above the nominal wind speed, only that fraction of the availablewind power is transformed to electric output power which corresponds tothe nominal power. This reduction of power production is, e.g., achievedby gradually changing the rotor-pitch angle towards the so-called flagposition. In other words, the wind turbine generator intentionally isnot operated at optimum efficiency. In some embodiments the wind turbinegenerator is also operated at a sub-optimal tip-speed ratio so as toreduce structural loads.

By contrast, during operation at partial load, i.e. at wind speed belowthe nominal wind speed, the wind turbine generator according to theembodiments is operated at optimum efficiency. For example, it isoperated with the aerodynamically optimal blade pitch angle andtip-speed ratio. Generally, the wind speed fluctuates in anunpredictable manner on a short-term basis (at the level of minutes downto a few seconds). When operating at partial load and with optimumefficiency the wind turbine generator according to the embodimentstransforms these wind-speed fluctuations in a nearly one-to-one mannerinto corresponding wind-caused fluctuations of the amount of real powerproduced and supplied to the electrical grid. Fluctuations in the winddirection may also contribute to the wind-caused fluctuations of theamount of real power produced result in and supplied to the electricalgrid because a wind-turbine's yaw adjustment mechanism is generally notable to immediately align the wind turbine generator's rotor axis to thewind direction. A misaligned rotor has a reduced efficiency so thatfluctuations in the wind direction are a further source of wind causedfluctuations of the amount of real power produced and supplied to theelectrical grid.

As mentioned at the outset, the consequence of fluctuating-powerproduction by wind turbine generators on the grid stability depends oncharacteristics of the grid. In a large, stable grid a power fluctuationby a wind turbine generator or wind power plant will not produce anysignificant response in the form of a frequency fluctuation. However, ina small isolated grid, or in weak grids, such a power fluctuation mayproduce a significant frequency fluctuation. A certain ability of thegrid to compensate imbalances of power production and compensation andregulate resulting frequency variations, i.e. a certain degree ofstiffness or weakness of the grid, is referred to as “grid-stability”.

The inventor has recognized that the grid-stability may vary over time,for example due to grid related failures, such as islanding of that partof the grid in which the wind turbine generator is located, due toprimary-producer failures, etc. The inventor has also recognized that adeterioration of the grid stability can be detected by monitoring e.g.frequency fluctuations on the grid. Moreover the inventor has recognizedthat it is desirable in the case of a deterioration of the gridstability conditions to limit the output-power fluctuations produced bythe wind turbine generator and supplied to the grid or if the windturbine generator has already operated with limited output-powerfluctuations before the deterioration occurred—to reduce the alreadyexisting fluctuation limit. “Reducing” the fluctuation limit meansmaking the limit stricter. By this measure, although the wind turbinegenerator according to the embodiments does not operate as a primarycontroller, it contributes to grid stability by reducing source-inducedfluctuations. However, limiting the output power fluctuations theaccumulated power output will generally be reduced and thereby theeffective efficiency of the wind turbine generator lowered. However, byrestricting this measure to situations in which the grid-stability is(temporarily) deteriorated, the loss of electric energy produced will belimited.

Some embodiments pertain to a control system arranged to control atleast one wind turbine which may include some, or all, of the windturbines of a whole wind park, in the manner describe above. The controlsystem may be an individual wind turbine controller, a wind power plantcontroller, a power plant controller or a controller at a higher levelin the grid and connected to the wind-turbine controller to sendlimit-fluctuation commands. The control system can be distributed, e.g.include controllers at the wind-park and the wind-turbine level orutility-grid level.

A variable speed wind turbine generator, which is used in at least oneof the above described embodiments and which is capable for beingconnected to an electrical grid is equipped with the control systemdescribed above. It comprises a rotor with a hub and at least one blademounted to the rotor as discussed above. The rotor is connected, forexample via a main shaft, to a generator for translating the torque ofthe rotor into electrical power. In some embodiments, a gearbox isinterconnected between the rotor and the generator in order to translatethe rotational speed of the rotor into a higher speed for the generator.

FIG. 1 shows, an exemplary variable-speed wind turbine generator (WPS) 1is one of a plurality of wind turbine generators of a wind power plant(WPP) 2.

It has a rotor 3 with a hub to which, e.g., three blades 4 are mounted.The pitch angle of the rotor blades 4 is variable by means of pitchactuators. The rotor 3 is supported by a nacelle 5 and drives agenerator 12 via a main shaft 8, a gearbox 10, and a high speed shaft11, where the structure and its components, e.g. WPS, is supported by atower 6 or base 6. This structure is exemplary; other embodiments, forexample, use a direct-drive 15 generator.

The generator 12 (e.g. Induction or synchronous generator) produceselectrical output power of a frequency related to the rotation speed ofthe rotor 3, which is converted to grid frequency (e.g. about 50 or 60Hz) by a converter 19. The voltage of the electric power thus producedis up-transformed by a transformer 9. The output of the transformer 9 isthe wind turbine generator's terminals 9 a. The electric power from thewind turbine generator 1 and from the other wind turbine generators ofthe wind power plant 2 is fed into a wind-park grid 18 (symbolized by“a” in FIG. 1). The wind power plant grid 18 is connected at a point ofcommon coupling 21 and an optional further step up transformer 22 to awind power plant external electrical utility grid 20. The grid 20 isequipped with regulation capacity against grid-frequency fluctuations,e.g. in the form of conventional producers which can increase and lowerproduction on a short-time scale to control frequency.

A control system includes a wind-turbine controller 13 and a wind powerplant controller 23. The wind-park controller 13 controls operation ofthe individual wind turbine generator 1, e.g. selects the full-load orpartial-load operation mode, depending i.a. on the current wind speed,causes, in the partial load mode, operation of the wind turbinegenerator at the optimal working point by adjusting the blade angle andcontrolling the tip speed ration to the aerodynamic optimum at thecurrent wind speed, and controls the converter 19 to produce electricityaccording to prescriptions of the wind-park-controller, e.g. aninstruction to provide a certain amount of reactive power in addition tothe active power, etc. The wind-park controller 13 uses different inputsignals to perform its control tasks, for example signals 16representing current wind conditions (e.g. from an anemometer 14 and awind vane 15), feed-back signals representing pitch angle, rotorposition, amplitudes and phases of the voltage and current at thegenerator 12 and the terminals 9 a, etc., and command signals from thewind-park controller 23. The wind-park controller 23 receives signalsrepresentative of the voltage, current and frequency at the point ofcommon coupling 21 (parameters which may be considered to represent thevoltage, current and frequency in the utility grid 20) and, optionally,receives information or command signals from the utility-grid provider(at “c” in FIG. 1). Based on some of these (and, optionally, further)input parameters the wind-park controller 23 monitors grid stabilityand, upon detection of a reduction of grid stability, commands thewind-turbine controllers 13 of the wind turbine generator 1 and theother wind turbine generators of the wind power plant 2 (at “b” inFIG. 1) to change operation by limiting fluctuations of the output powersupplied. Upon receipt of such a command the wind-turbine controller 13,upon increase of the wind speed, cuts the high-output peak which wouldthen be produced in normal partial-load operation with maximumefficiency, e.g., by adjusting the blade-pitch angle towards the flagposition, to comply with the wind-park controller's limit-fluctuationcommand. Thus, in the exemplary embodiment of FIG. 1 the control task ofthe control system to limit output fluctuations is shared by thewind-park controller 23 and the wind-turbine controller 13. In otherembodiments this control task is performed by the wind turbinecontroller 13 alone; in those embodiments, the “control system” isrepresented just by the wind turbine controller 13, without a wind-parkcontroller.

Although the wind turbine generator 1 shown in FIG. 1 is expected tohave three blades 4, it should be noted that a wind turbine generatormay have different number of blades. It is common to find wind turbinegenerators having two to four blades. The wind turbine generator 1 shownin FIG. 1 is a Horizontal Axis Wind Turbine (HAWT) as the rotor 4rotates about a horizontal axis. It should be noted that the rotor 4 mayrotate about a vertical axis. Such a wind turbine generators having itsrotor rotate about the vertical axis is known as a Vertical Axis WindTurbine (VAWT). The embodiments described henceforth are not limited toHAWT having 3 blades. They may be implemented in both HAWT and VAWT, andhaving any number of blades 4 in the rotor 4.

In order to ensure a stable electrical grid, various requirements hasbeen set for grid connection of wind turbine generators. Topics of thegrid codes may vary, but often there are requirement for Voltage controland Frequency control. Different grid codes may require differentcharacteristics for frequency response from wind power. Each grid codemay have pros and cons, depending on the characteristics and needs ofthe particular power system.

An isolated power system with high wind power penetration is known tocause a large impact on system frequency. Thus, two main sources offrequency deviations are identified:

-   -   1—Frequency deviations caused by grid events (normal operation        or disturbances), independently of the controlled wind power        production, e.g. load changes, generation changes or other        fluctuations from “uncontrolled” wind power sources in the power        system.    -   2—Frequency deviations caused by controllable wind power        fluctuations.

By implementing a first set of wind frequency control requirements in apower system with large amounts of wind power, some issues might begenerated during particular grid-wind situations. Therefore thefollowing situations may arrive:

-   -   Situations of high-frequency events followed by an increase in        P_(Ava) (actual available power): The increase in P_(Ava)        produces an increase in generated power, even if the generated        power was automatically reduced by the high-frequency event. As        result, the high grid frequency may increase even further. This        behavior may be avoided.    -   Situations of low-frequency events followed by a reduction in        P_(Ava): The reduction in P_(Ava) produces a reduction in        generated power, even if the generated power was automatically        increased by the low-frequency event and enough deload        (curtailment) was previously applied. As result, the low grid        frequency may decrease even further. This behavior may be        avoided as long as enough deload is previously allocated.

By implementing a second set of wind frequency control requirements in apower system with large amounts of wind power, no critical issues whereidentified, but some characteristics to be considered, such as:

-   -   Situations of high-frequency events followed by a reduction in        P_(Ava): If in FSM (Frequency Sensitive Mode), the reduction in        P_(Ava) will not reduce the generated power even if the        generated power was automatically reduced by the high-frequency        event; unless the P_(Ava) Ava decreases beyond the curtailed        value plus the generation reduction. Therefore the grid        frequency will not be further benefited by this _(PAva)        reduction. This behavior is different from what would be the        behavior with the first set of requirements.    -   Situations of low-frequency events and increase in P_(Ava): If        in FSM, the increase in P_(Ava) will not increase the generated        power even if the generated power was automatically increased by        the low-frequency event. Therefore the grid frequency will not        be further benefited by this P_(Ava) increase. This behavior is        also different from what would be the behavior with the first        set of requirement.    -   Situations of low-frequency events and decrease in P_(Ava): If        in FSM, an amount of deload needs to be applied in advance. The        further reduction of P_(Ava) will reduce the margin of power        reserve, therefore in low-frequency events the wind plant (or        wind power system) may run out of reserve for frequency        response. This behavior is not possible with the first set of        requirement as it always consider the available power for        allocating reserve (deloading i.e. curtailment).

In this present invention, a particular wind power frequency responsehas been identified as suitable for isolated power systems with highwind power penetration. Here the system frequency is stable as long asthe overall powers are balanced at system level.

The present invention is a control method with a frequency responsemixing both the first and second set requirement methodologies, thebeneficial part from each one, is elaborated. Its response depends onthe characteristics of the frequency deviation, i.e. high-frequencyevents or low-frequency events and it is described as follows:

-   -   High-Frequency Events:    -   The new output of wind power production will be the minimum        value of the following:        -   P_(RefA): Wind power demand will decrease an amount ΔP_(FC)            below the prevent electrical output and proportionally to            the grid frequency deviation.        -   P_(RefB): Wind power demand will decrease an amount □PFC            below the prevent curtailed power (related to PAva)            proportionally to the grid frequency deviation.    -   Low-Frequency Events:    -   The new output of wind power production will be the maximum        value of the following        -   P_(RefA): Wind power demand will increase an amount ΔP_(FC)            above the pre-event electrical output and proportionally to            the grid frequency deviation.        -   P_(RefB): Wind power demand will increase an amount ΔP_(FC)            above the pre-event curtailed power (related to P_(Ava))            proportionally to 1 the grid frequency deviation.

In order to respond to low frequency events with wind power, an amountof power reserve has to be allocated. This power reserve can beallocated by a wind power plant controller (system level). Inembodiments of the present invention wind power plant controller can besplit among decentralized controllers, such as clusters, power plant orwind turbine generator level).

Nevertheless, the wind frequency response described in this section canwork independently of the implementation level, but parameters should beadjusted accordingly. Thus, for simplification in the analysis we assumehere that it is implemented at system level. The reserve of wind poweris achieved by curtailing the generation an amount P_(ReserveWPS) belowthe available power. FIG. 3 shows the introduction of P_(ReserveWPS).FIG. 2 shows the coexistence of P_(ReserveWPS) with the fluctuationslimitation functionality (ΔP_(WindFLuct)). Both functionalities areensured by the controller.

The value ΔP_(FC) in pu is calculated independently of the availablepower as follows:

$\begin{matrix}{{\Delta\;{P_{FC}\lbrack{pu}\rbrack}} = \left\{ {\begin{matrix}{{\frac{1}{R_{HF}}\frac{\left( {{\Delta\; f} - {Thr}_{HF}} \right)}{f_{0}}},} & {\forall{{\Delta\; f} > {Thr}_{HF}}} \\{{\frac{1}{R_{LF}}\frac{\left( {{\Delta\; f} - {Thr}_{LF}} \right)}{f_{0}}},} & {\forall{{\Delta\; f} < {Thr}_{LF}}} \\{0,} & {{Thr}_{LF} < {\Delta\; f} < {Thr}_{HF}}\end{matrix},} \right.} & (1)\end{matrix}$

where R_(HF) and R_(LF) are proportionality constants (droop)respectively for high-frequencies and low-frequencies, Thr_(HF) andThr_(LF) are threshold values respectively for high-frequencies andlow-frequencies. FIG. 4 shows the wind power regulation characteristicand FIG. 5 shows the block diagram for its calculation according to Eq.(1).

When the grid frequency deviation goes beyond the thresholds, thefrequency control is activated, the actual electrical production ismemorized as P_(elWPS0) and the actual available power is memorized asP_(AvaWSP0). The pre-event curtailed power is given by:P _(DeloadWPS0) =P _(AvaWPS) −P _(ReserveWPS0) =P _(AvaWPS)−(P_(AvaWPS0) =P _(elWPS0))  (2)

The ΔP_(FC) is then applied to the P_(elWPS0) to get P_(RefA) and to theP_(DeloadWPS0) to get the P_(RefB). Then the demand on wind powerproduction is chosen according to Eq. (3).

$\begin{matrix}{P_{DemandWPS} = \left\{ \begin{matrix}P_{{refWPS},} & {{for}\mspace{14mu}{normal}\mspace{14mu}{frequencies}} \\{{\min\left\{ {\left( {P_{{DeloadWPS}\; 0} \pm {\Delta\; P_{FC}}} \right),\left( {P_{{el}\; 0} \pm {\Delta\; P_{FC}}} \right)} \right\}},} & {{for}\mspace{14mu}{high}\mspace{14mu}{frequencies}} \\{{\max\left\{ {\left( {P_{{DeloadWPS}\; 0} \pm {\Delta\; P_{FC}}} \right),\left( {P_{{el}\; 0} \pm {\Delta\; P_{FC}}} \right)} \right\}},} & {{for}\mspace{14mu}{low}\mspace{14mu}{frequencies}}\end{matrix} \right.} & (3)\end{matrix}$

FIG. 6 shows the block diagrams for the memory functions as in Eq. (2).FIG. 7 shows the calculation of the demand for the wind powerproduction, similar to Eq. (3).

FIG. 8 shows an example of the generation response to a fictitioushigh-frequency event and FIG. 9 shows similar to a low-frequency event.In FIG. 8 if the P_(Ava) increases, the generation can never be higherthan P_(RefA); but if P_(Ava) decreases, the generation can follow itwith _(PRefB) and eventually bring the grid frequency down. In FIG. 8 ifthe P_(Ava) decreases, the generation is not lower than _(PRefA) (aslong as enough reserve is available); but if P_(Ava) increases, thegeneration can follow it with P_(RefB) and eventually bring the gridfrequency up. As grid frequency fluctuations are also generated by windpower fluctuations, a frequency controller of these characteristics forwind power production introduces a self-stabilizing mechanism.

FIG. 10 shows a table with different examples of frequency ranges forvarious grid requirements as function of the grid system state. Thehigher the frequency range is the more severe the grid system is in.

FIG. 11 shows a flow chart of a method according to the invention foroperating a wind power plant, with at least one wind turbine generatorconnected to an electrical grid, Step 900 is monitoring a frequencyparameter and comparing it with a frequency set point, as to select alow or high frequency event, and step 901 is determining a powerreference to the a least one wind turbine generator, based on thefrequency parameter, wherein:

-   -   In case a high frequency event is detected, the power reference        is        -   determined as a minimum of a selection of at least a first            and a second minimum power reference,    -   In case a low frequency event is detected, the power reference        is        -   determined as a maximum of a selection of at least a first            and a second maximum power reference,    -   and step 902 is dispatching the power reference to the at least        one wind turbine generator.

In embodiments limiting the active-power fluctuations is, e.g., achievedby means of blade pitch adjustment. In some embodiments active-powerfluctuations are also limited electrically, by corresponding control ofthe wind turbine generator's electric-power converter. However, thelater results in imbalance between the amount of wind power convertedinto mechanical power of the wind turbine generator's rotor and theelectric output power which, e.g., results in acceleration of the rotor.

Therefore, in some embodiments electrically limiting power is onlyperformed in combination with blade-pitch adjustment to cope withwind-speed transients For example, when the wind speed rises faster thatthe pitch can be adjusted to compensate for the wind speed rise, theoutput power is first limited electrically and, once the blades havebeen pitched to their new pitch angle, is then limited by the pitchadjustment.

The present description focuses on limiting, or further limiting, theoutput-power fluctuations. However, the invention also goes in the otherdirection, that is relaxing or cancelling the limit, in an analogousmanner. That is to say, upon detection of increased grid stability, theoperation of the wind turbine generator is changed by cancelling orrelaxing a previously set fluctuation limit.

The monitoring and limit-adjustment function is a self-diagnosis andself-adjustment function performed by a control system at the level ofindividual wind turbine generators, or at the level of a wind powerplant, or at a higher level in the utility grid. The control system canalso be distributed, e.g. include controllers at the wind-park and thewind-turbine level.

In some embodiments the frequency range covered by grid-frequencyfluctuations is permanently determined, and a variation of the gridfrequency outside an allowed-frequency range F_(max)/F_(min) (between anallowed maximum frequency F_(max) and an allowed minimum frequencyF_(min)) range is considered to be a detection of a reduced gridstability condition, i.e. a frequency dead band. Alternatively or inaddition, the variance of the grid frequency is permanently determined,and a rise beyond a variance threshold is considered to be a detectionof a reduced grid stability condition. The allowed fluctuation of thewind turbine generator's or wind power plant's power output is thenreduced.

In some embodiments monitoring whether the grid-frequency fluctuationsare within the allowed-frequency range, or whether their variance isbelow the variance threshold is performed in an absolute manner, i.e.without taking into account any correlation of the grid frequency andthe output power produced by the wind turbine generator or wind powerplant.

However, correlation-less monitoring grid-frequency fluctuations issomewhat unspecific in the sense that it is not ensured that thefluctuation of the wind-turbine or wind power plant considered actuallycontributes to the grid-frequency fluctuations observed. Therefore, inthese embodiments the reduction of the fluctuation limit might be invain, and would only produce costs (by the reduction of the accumulatedpower output caused by it). Therefore, in other embodiments themonitoring of grid stability comprises determining a correlation betweenpower supplied to the electrical grid and grid frequency. Correlationmeans that if the power output increases the grid frequency alsoincreases. The grid frequency is, e.g. measured at the wind turbinegenerator's terminals or at a wind power plant's point of coupling tothe grid. If, however, no increase of the grid frequency is observedupon increase of the output power there is no correlation. Actually,“correlation” can be a continuous parameter measuring the degree ofcoincidence between the output power increase and the grid frequencyrise.

In some of the embodiments, the bigger is the correlation thusdetermined, the smaller is the grid stability detected. To be consideredas an indicator for reduced grid stability, a rise of the correlationhas to be significant in some embodiments, e.g. the rise has to exceed amaximum-acceptable correlation threshold. The allowed fluctuation of thewind turbine generator's output power is then reduced. Linking thereduction of the fluctuation limit to the observed correlation betweenoutput-power fluctuations and grid-frequency fluctuations ensures thatthe reduction of the output-power fluctuation limit actually contributesto reduction of the grid-frequency fluctuations.

In some embodiments, the correlation information is used to determinewhether the variation of the grid frequency extends beyond theallowed-frequency range F_(max)/F_(min) or whether the frequencyvariance exceeds the variance limit, by taking only those peaks (ordips) in the grid frequency into account which can be attributed to acorresponding peak (or dip) of the output power of the wind turbinegenerator or wind power plant considered. This is taking into accountcorrelation on a peak-by-peak basis.

In other embodiments the correlation information is used for the samepurpose more globally, (not peak-by-peak) by multiplying theuncorrelated fluctuation amplitude by the magnitude of the correlation,which may be a number between 0 and 1 (or by multiplying theuncorrelated frequency variance by the square of the fluctuation).“Diluting” the observed uncorrelated fluctuation amplitude or variancein this manner takes into account that only a fraction of the observeduncorrelated fluctuation amplitude or variance is due the output-powerfluctuations of the wind turbine generator or wind power plantconsidered.

A prerequisite of such a correlation measurement is that there is avariation of the wind turbine generator's output power. In someembodiments, also referred to as “passive-variation embodiments”, use ismade of the output power variations caused by the natural wind-speedvariations. These passive-power variations are tracked and correlatedwith the measured grid frequency.

In some embodiments the limit on power fluctuations is chosen such thatthe grid-frequency fluctuations caused by the supply of power aremaintained inside the range F_(max)/F_(min) or the variance ofgrid-frequency fluctuations caused by the supply of power is maintainedbelow the variance limit.

In some of these embodiments the entire grid-frequency fluctuation(including the contribution not caused by the wind turbine or wind parkconsidered) is to be maintained inside the range F_(max)/F_(min) orbelow the variance threshold, while in other embodiments only thatfraction of the grid-frequency fluctuations which is caused by the powersupply of the wind turbine or wind park considered is maintained insidethe range F_(max)/F_(min) or below the variance threshold.

In some of the embodiments in which the (entire or fractional)grid-frequency fluctuation is to be maintained inside the rangeF_(max)/F_(min) or below the variance threshold, the fluctuation limitto the output power is continuously adjusted to that extent of limitjust needed to keep the grid frequency inside the range F_(max)/F_(min)or the variance below the variance threshold. That means that the powerproduction by the wind turbine or wind park is maximized by letting theoutput power fluctuate, but the fluctuation is limited, or modulated, ifthe grid frequency goes beyond F_(max)/F_(min) Thus, the objective thecontinuous adjustment is to avoid the grid frequency to go out of theF_(max)/F_(min) range without losing more power production thannecessary.

In some embodiments the operation of the wind turbine is automaticallyswitched between two discrete operation modes, that is to say from anormal-operation mode (i.e. a mode with no power-fluctuation limit, orwith a relatively relaxed power-fluctuation limit) to a reducedfluctuation mode (in which the power fluctuation limit is activated).The automatic mode switch from the normal-operation mode to thereduced-fluctuation mode is triggered, in some of these embodiments, bydetection of a reduction of the grid stability beyond a lowermode-switch threshold. Switching from the reduced-fluctuation mode backto the normal-operation mode can likewise be triggered by detection ofan increase of the grid stability beyond an upper mode-switch threshold.

In some of the mode-switching embodiments the reduced-fluctuation modeis maintained a minimum time interval before the mode can switch back tothe normal-operation mode. By this measure too frequent mode switchingcan be avoided. Moreover, there may be a contractual agreement with thegrid provider according to which the wind-power producer is committed tosupply output power with strongly limited output-power fluctuationduring a predetermined time interval, say 15 min. The wind-powerproducer can be compensated for the production loss suffered due to this(exemplary) 15-min. period of smooth output power supply.

In some of the mode-switching embodiments the power-fluctuation limit iskept constant during the reduced-fluctuation mode. Constancy of thepower-fluctuation limit refers to the width of the limit relative to amean output power; it does not necessarily mean that the absolute valuesof the upper and lower power limits are kept constant. In someembodiments the limit is relative a mean value of the power produced.For example, if the mean power produced increases with time, theabsolute values of the upper and lower power-fluctuation limits willalso increase.

In other mode-switching embodiments the fluctuation limit is alsoadjusted to avoid the grid frequency to go out of the F_(max)/F_(min)range without losing more power production than necessary, as wasdescribed above. This adjustment may be stepwise (an setting adjusted atthe beginning of mode switch and then kept constant for a certain periodof time) or continuous. Thus, the output-power adjustment to keep thegrid frequency inside the range F_(max)/F_(min) or below the variancethreshold, to that extent of limit just needed to keep the gridfrequency inside the range F_(max)/F_(min) or the variance below thevariance threshold applies to both the continuous-adjustment embodimentsand the mode-switching embodiments.

It has already been mentioned that limiting power fluctuation may resultin a loss of accumulated power. A loss of accumulated power could beavoided if not only peaks of the output power (“positive fluctuations”)were cut, but also dips of the output power (“negative fluctuations”)were lifted, or filled up, in a symmetric manner. However, in someembodiments the wind turbine is at its optimal working point duringnormal-mode operation, which does not allow any increase of the outputpower. Therefore, limiting output fluctuations is rather performed in anasymmetric manner, by cutting the output power during positivefluctuations (cutting high output peaks), without (or withoutsignificantly) lifting the relative output power during negativefluctuations. As explained above, cutting the output power duringpositive fluctuations is, e.g., achieved by a corresponding adjustmentof the blade-pitch angle towards the flag position.

The strictness of the limit on output-power fluctuations, and/or theposition of the threshold which has to be exceeded by the gridinstability so that mode switching is performed, may also depend onother factors than the monitored grid stability.

For example, in some of the embodiments a wind forecast is used to varythe fluctuation limit, e.g. to make it stricter when the forecastpredicts increased wind-power fluctuation. Moreover, in mode-switchingembodiments the mode-switch threshold may be varied in response to thewind forecast. For example, the threshold may be varied upon a forecastof increased wind power fluctuation such that switching from thenormal-operation mode to the reduced fluctuation mode is alreadytriggered at a less pronounced reduction of the grid stability.

Similarly, in other embodiments an expectation of power consumption inthe electrical grid is used to vary the fluctuation limit, or to varythe mode-switch threshold. For example, a power consumption expectationgiving rise to expectation of increased grid-frequency fluctuation mayrender the fluctuation limit stricter, or modify the mode-switchthreshold such that switching from the normal-operation mode to thereduced-fluctuation mode is already triggered at a less pronouncedreduction of the grid stability.

The term wind turbine generator, WPS is to be understood both as asingle wind turbine generator according to FIG. 1, but in someembodiments it may also be a group of wind turbine generator accordingto FIG. 1 connected at a point of common coupling, thereby, from a powersystem operator, seen as one source of wind power from one location.

The term wind power plant, WPGS may in some embodiments be a single windpower plant with a plurality of wind turbine generators according toFIG. 1. In other embodiments wind power plant is to be understood as anaggregation of wind power plants located at different geographicallocation, either adjacent to each other or remote from each other, butall of them are controllable by means of the dispatcher of the presentinvention.

Although the present invention has been described in connection with thespecified embodiments, it should not be construed as being in any waylimited to the presented examples. The scope of the present invention isto be interpreted in the light of the accompanying claim set. In thecontext of the claims, the terms “comprising” or “comprises” do notexclude other possible elements or steps. Also, the mentioning ofreferences such as “a” or “an” etc. should not be construed as excludinga plurality. The use of reference signs in the claims with respect toelements indicated in the figures shall also not be construed aslimiting the scope of the invention. Furthermore, individual featuresmentioned in different claims, may possibly be advantageously combined,and the mentioning of these features in different claims does notexclude that a combination of features is not possible and advantageous.

Any range or device value given herein may be extended or alteredwithout losing the effect sought, as will be apparent to the skilledperson.

It will be understood that the benefits and advantages described abovemay relate to one embodiment or may relate to several embodiments. Itwill further be understood that reference to ‘an’ item refer to one ormore of those items.

It will be understood that the above description of a preferredembodiment is given by way of example only and that variousmodifications may be made by those skilled in the art. The abovespecification, examples and data provide a complete description of thestructure and use of exemplary embodiments of the invention. Althoughvarious embodiments of the invention have been described above with acertain degree of particularity, or with reference to one or moreindividual embodiments, those skilled in the art could make numerousalterations to the disclosed embodiments without departing from thespirit or scope of this invention.

What is claimed is:
 1. A method of operating a wind power plant, with atleast one wind turbine generator connected to an electrical grid,wherein the method comprises: monitoring a frequency parameter andcomparing it with a frequency set point, as to select a low or highfrequency event; determining a power reference to the at least one windturbine generator, based on the frequency parameter, wherein: inresponse to detection of a high frequency event, the power reference isdetermined as a minimum of a selection of at least a first and a secondminimum power reference, wherein the first minimum power reference is apre-event electrical power output subtracted a power amountproportionally to the grid frequency deviation (ΔPFC) and wherein thesecond minimum power reference is a pre-event curtailed electrical powerreference subtracted a power amount proportionally to the grid frequencydeviation (ΔPFC), in response to detection of a low frequency event, thepower reference is determined as a maximum of a selection of at least afirst and a second maximum power reference, wherein the first maximumpower reference is a summation of a pre-event electrical power outputand a power amount proportionally to the grid frequency deviation(ΔPFC), and wherein the second maximum power reference is a summation ofa curtailed electrical power output and a power amount proportionally tothe grid frequency deviation (ΔPFC); and dispatching the power referenceto the at least one wind turbine generator; and adjusting a power outputof the at least one wind turbine generator using the power reference. 2.The method according to claim 1, wherein the curtailed electrical powerreference is a power reserve.
 3. The method according to claim 2,wherein the power reserve is combined with a limitation function, as toreduce power fluctuations from the wind power plant.
 4. The methodaccording to claim 1, wherein the power amount proportionally to thegrid frequency deviation, ΔP_(FC) further comprises a thresholdcomponent.
 5. The method according to claim 4, wherein the thresholdcomponent consists of a low frequency threshold value and a highfrequency threshold value.
 6. The method according to claim 1, whereinthe adjustment reduces at least one adverse effect associated with apower fluctuation resulting from a change in stability of the electricgrid.
 7. A wind power plant, comprising at least one wind turbinegenerator connected to an electrical grid, and a power plant controller,wherein the power plant controller is arranged to monitor a frequencyparameter and compare it with a frequency set point, as to select a lowor high frequency event, and said power plant controller is arranged todetermine a power reference to the at least one wind turbine generator,based on the frequency parameter, wherein: in response to detection of ahigh frequency event, the power reference is determined as a minimum ofa selection of at least a first and a second minimum power reference,wherein the first minimum power reference is a pre-event electricalpower output subtracted a power amount proportionally to a gridfrequency deviation (ΔPFC) and wherein the second minimum powerreference is a pre-event curtailed electrical power reference subtracteda power amount proportionally to the grid frequency deviation (ΔPFC), inresponse to detection of a low frequency event, the power reference isdetermined as a maximum of a selection of at least a first and a secondmaximum power reference, wherein the first maximum power reference is asummation of a pre-event electrical power output and a power amountproportionally to the grid frequency deviation (ΔPFC), and wherein thesecond maximum power reference is a summation of a curtailed electricalpower output and a power amount proportionally to the grid frequencydeviation (ΔPFC); and and said power plant controller is arranged todispatch the power reference to the at least one wind turbine generator;and adjust a new power output of the at least one wind turbine generatorusing the power reference.
 8. A wind power plant, with at least one windturbine generator connected to an electrical grid, and a wind powerplant controller configured to carry out the following: monitoring afrequency parameter and comparing it with a frequency set point, as toselect a low or high frequency event; determining a power reference tothe at least one wind turbine generator, based on the frequencyparameter, wherein: in response to detection of a high frequency event,the power reference is determined as a minimum of a selection of atleast a first and a second minimum power reference, wherein the firstminimum power reference is a pre-event electrical power outputsubtracted a power amount proportionally to a grid frequency deviation(ΔPFC) and wherein the second minimum power reference is a pre-eventcurtailed electrical power reference subtracted a power amountproportionally to the grid frequency deviation (ΔPFC), in response todetection of a low frequency event, the power reference is determined asa maximum of a selection of at least a first and a second maximum powerreference, wherein the first maximum power reference is a summation of apre-event electrical power output and a power amount proportionally tothe grid frequency deviation (ΔPFC), and wherein the second maximumpower reference is a summation of a curtailed electrical power outputand a power amount proportionally to the grid frequency deviation(ΔPFC); dispatching the power reference to the at least one wind turbinegenerator; and adjusting a power output of the at least one wind turbinegenerator using the power reference.
 9. The wind power plant accordingto claim 8, wherein the adjustment reduces at least one adverse effectassociated with a power fluctuation resulting from a change in stabilityof the electric grid.